This invention relates to semiconductor transducers in general and more particularly to such transducer assemblies which employ piezoresistive elements, which elements are dielectrically isolated from their respective diaphragms while utilizing high concentration doped layers.
Presently, semiconductor transducers, because of their relatively small dimensions, are finding wide use in a variety of applications. These devices, although being extremely small, possess high reliability and increased response. Such devices are widely utilized in the medical field, the electronics field, and various other fields for making pressure measurements. It is, of course, understood that for many stringent applications, great care has to be taken in providing a small and reliable device while making the device as sensitive as possible. In regard to this, certain applications require that the piezoresistive elements be dielectrically isolated from the diaphragm on which they are located. The piezoresistive transducer employs a silicon resistive element, which resistance varies according to the intensity or magnitude of an applied force upon an associated diaphragm. Such resistors comprise serpentine or tortuous line patterns. As is known, the greater the length of the pattern, the larger the resistor. In any event, it is a desire to form a long line pattern in a relatively small space, which line pattern possesses a relatively narrow width. It is further desirable to provide such a pattern, which is essentially a fine line pattern, in a small space where further the piezoresistive element is dielectrically isolated from the semiconductor diaphragm that it is coupled to. The piezoresistive element, as indicated, varies resistance according to the intensity of an applied force. The force is usually applied to a relatively thin semiconductor diaphragm or metal diaphragm which is a membrane-like structure and to which the semiconductor gauge is mounted or otherwise diffused therein. A force applied to the diaphragm serves to deflect the diaphragm and hence causes the associated piezoresistive element to vary resistance in accordance with the deflection. The force being measured is transferred through the diaphragm to the strain responsive element, causing the element to expand or compress. This produces a change in the resistance of the element. Such elements are conventionally arranged as Wheatstone bridge circuits with one to four of the bridge legs being active. Essentially, it is an increased desire of the prior art to produce a device that is relatively small, sensitive and which possesses a fairly large resistance in a relatively small area as well as being dielectrically isolated from the diaphragm. There are techniques employed and known in the prior art for the fabrication of dielectrically isolated transducer elements. See, for example, U.S. Pat. No. 3,951,707 by A. D. Kurtz et al. issued on Apr. 20, 1976 and assigned to the assignee herein, which patent is entitled "Method for Fabricating Glassbacked Transducers and Glassbacked Structures." In this technique there is disclosed a method of fabricating a thin ribbon piezoresistive bridge which eventually is to be secured to a thin glass wafer or the diaphragm structure. See also, U.S. Pat. No. 3,800,264 entitled "High Temperature Transducers and Housing Including Fabrication Methods" issued on Mar. 26, 1974 to A. D. Kurtz et al. and assigned to the assignee herein. This patent describes a dielectrically isolated pressure transducer which includes a silicon diaphragm having on a surface a piezoresistive sensor which is isolated from the diaphragm surface by a dielectric insulator.
See also U.S. Pat. No. 4,510,621 issued on Apr. 16, 1985 and entitled "Dielectrically Isolated Transducer Employing Single Crystal Strain Gages" by A. D. Kurtz et al. and assigned to the assignee herein. This patent shows a single crystal semiconductor diaphragm dielectrically isolated by a layer of silicon dioxide from a single crystal gage configuration. The methods employ high dose oxygen which is ion implanted into a monocrystalline wafer to form a buried layer of silicon dioxide with the top surface of the wafer being monocrystalline silicon.
U.S. Pat. No. 4,406,992 entitled "Semiconductor Pressure Transducers or Other Products Employing Layers of Single Crystal Silicon" issued on Sept. 27, 1983 to A. D. Kurtz et al. and assigned to the assignee herein. This patent shows a single crystal silicon sensor positioned on a single crystal diaphragm and isolated by a layer of silicon dioxide.
Apart from the aspect of dielectrically isolating the transducer, it is a further desire to produce a single crystal sensor which is dielectrically isolated from a single crystal substrate. In providing a single crystal substrate, one can now have a semiconductor diaphragm which is single crystal, thus avoiding many of the disadvantages associated with polycrystalline diaphragms as well as with polycrystalline sensor elements.
It is further desirable to provide a piezoresistive transducer which has a very small temperature coefficient and which possesses a narrow line width resistive pattern to enable large value resistors to be fabricated on relatively small diaphragms.
It is also desirable to provide a semiconductor transducer whereby the sensor device is dielectrically isolated from the structure.
L. Bruce Wilner, is an article entitled "Miniature Pressure Transducers For Use to 300.degree. C." published in the ISA JOURNAL, 1982 ISBN 0-87664-689-5, has described a method of providing a structure with some of these desirable features but the method of fabrication and the resulting structure has a number of shortcomings. Wilner teaches that one should take a first wafer which is the carrier wafer and first form diaphragm apertures on one surface and oxidize the other surface. He then teaches that one should take a second wafer which has been diffused to a relatively high doping of boron. He then suggests that the two wafers are joined "by an oxide diffusion bond in which a very thin film of low-melting oxide on one wafer wets the interface between the wafers and at very high temperatures dissolves and diffuses into a thicker film of silicon dioxide on the second, or carrier wafer".
This process requires very high temperatures, 1000.degree. C. or more, high pressures and long times and results in a structure with doubtful adherence between the two wafers. Moreover, because of the long times and temperatures required, redistribution of the diffused layer into the sacrificial wafer may occur. In any event, Wilner then indicates that the N type material should be removed from the sacrificial wafer and an appropriate gage pattern formed on the resulting p type layer now adhered to the carrier wafer. This requires an etching operation to remove the excess p type material thus forming a serpentine gage or sensor pattern.
An object of this invention is to overcome many of the difficult costly and critical features of Wilner's process. For instance, it would be desirable to form the bond between sacrificial and carrier wafer in seconds rather than days at a low temperature of 350.degree. C., rather than high temperature, under low rather than high pressures.
It is also an object of this invention to produce very fine line geometry such as obtained with conventional oxide masking rather than that which is obtained with chemical removal.
It is a further object of this invention to preserve the very highly doped sensor configuration rather than to allow it to be degraded by further high temperature processing.
It is a further object to provide a semiconductor transducer which possesses a small temperature coefficient over a large operating range and which possesses narrow line width resistors to enable the fabrication of large resistance values on small diaphragms.